Highly deviated or horizontal wells are commonly drilled for oil and gas production. As used herein, the term “horizontal” refers to not only wells with truly horizontal sections, but also to wells that are highly deviated. In one type of horizontal well completion, the operator installs and cements a casing or liner that extends to the total depth of the well. Normally, the term “casing” refers to conduit that extends back to the surface wellhead, and “liner” refers to conduit that has its upper end supported near the lower end of a first string of casing. These terms will be used interchangeably herein to refer to a conduit in a well that is cemented in place, whether its upper end extends to the surface or just to the lower end of a first string of casing.
After cementing the casing, the operator perforates through the casing into the producing formation. The operator may then perform other operations, such as hydraulic fracturing or dispensing acid or other chemicals into the producing formation. Normally, the operator installs a string of production tubing in the casing for the production flow.
Even though wells may be fairly close to each other, producing formations often vary in characteristics from one well to another, such as thickness, depth, porosity, water content, permeability and the like. Consequently, it is useful to have a survey or log made of the well before it is cased to provide the characteristics of the producing formation. In highly deviated and horizontal wells, logging can be made while drilling using measuring while drilling techniques.
After cementing, it is also useful for the operator to perform another survey of the well. Because of the casing, the cased-hole log differs from an open-hole survey. By using tools such as ones that measure natural gamma rays emitted by earth formations, the operator will be able to discern the same formations previously noted during the open-hole survey. The operator uses this information to determine precisely where to perforate. Even without an open-hole log, a cased-hole survey provides important information to the operator.
In a vertical or even a moderately deviated well, the operator can run a cased-hole log before perforating by lowering a surveying instrument on a wireline into the casing and making the survey either while running-in or retrieving. Logging a cased horizontal well presents a problem, because gravity won't pull the tool down. One approach has been to mount to the instrument a tractor with motor-driven wheels or tracks. Generally, these logging procedures are expensive and slow. Also, high voltages are typically required, which can be detrimental to the wireline.
Surveying instruments have been pumped down wells in the prior art. An annular piston is mounted to the instrument assembly for sealingly engaging the conduit. This type of operation requires a flow path for displaced fluid below the piston as the instrument moves downward. In the prior art, the flow path typically comprises an open annulus surrounding the conduit containing the instrument. In a cased horizontal well, there is no open annulus surrounding the casing and no place for displaced fluid. Consequently, pump-down logging is normally not performed on horizontal wells.